A simple but very functional burn card. I find that a medium sized circle and a right triangle cover about 99% of my burning needs, but I have included the Blender file here if you want to take a "blank" burn card and put in whatever shape you want.

Opaque black filament recommended for the card itself. I use Jarees Matte Black PETG, which I find to be excellent for photography-related prints. It's easy to print, inexpensive, reaches 100% effective opacity in fewer layers than other black filaments, and has a more matte/less reflective finish than any other filament I've ever tried.

The white layers can be printed separately and glued into the provided indent in the card with a small amount of superglue or epoxy. The idea is that the white layers face up during a burn in, and make it easy to see the projected image from the enlarger. The black layers face down and provide the needed opacity as well as minimizing reflected light that could fog the paper. I'm using Hatchbox white PLA here.

Everything is optimized to print at a 0.2mm layer height with no supports. I would recommend a minimum of 4 bottom/top layers for the black parts to guarantee opacity. Infill % doesn't matter, use whatever you want. Cubic infill is theoretically best for blocking light, but it's probably not important if you have enough bottom and top layers.